1. Field of the Invention
The present invention relates to lithium ion secondary batteries to achieve both of high volumetric energy density and high volumetric power density.
2. Background Art
Lithium ion secondary batteries have been widely used mainly as power supplies for portable equipment such as a VTR camera, a laptop computer and a mobile phone. In recent years they are used in a variety of fields including video game machines, electrical power tools and electric bicycles.
Particularly in the automobile industry, electric vehicles with zero emission powered by a battery only as well as Hybrid Electric Vehicles (HEVs) and Plug-in Hybrid Electric Vehicles (PHEVs) powered by both of an internal-combustion engine and a battery are developed on a full scale basis to cope with environmental issues and partially have reached the stage of practical use. Therefore batteries as power supplies also are being developed actively.
In HEVs and PHEVs configured to assist the accelerating power of a vehicle with a motor, the battery thereof repeats instantaneous charge and discharge at a large current value of 10 C or higher while the vehicle is moving in most cases, although depending on a control method of the system. Therefore, the battery in such a vehicle is required to have high volumetric power density. Further, in order to downsize a battery or lengthen a traveling distance of a PHEV vehicle with one charge, the battery is required to have high volumetric energy density as well as high volumetric power density.
The charge/discharge reaction of a lithium ion secondary battery is performed in the vicinity of an interface between active material particles in electrodes and electrolyte solution flowing through a vacancy volume in the electrodes. For higher volumetric power density of a lithium ion secondary battery, the battery is required to have a sufficient reactive area between these active material particles and electrolyte solution and have a sufficient amount of electrolyte solution existing in the vicinity of active material particles secured to supply the electrolyte solution to a reactive face between the active material particles and the electrolyte.
Meanwhile, for higher volumetric energy density, the amount of active material particles incorporated into electrodes has to be increased. As one method to increase the amount of active material particles incorporated, the particle diameter and the particle shape of active material particles are controlled to minimize a vacancy volume between mixed particles for high-density filling so that more active material particles can be put in a fixed area (Patent Document 1: JP Patent Publication (Kokai) No. 2004-192846 A).